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---
layout: blog
title: "Introducing Volume Group Snapshot"
date: 2023-05-08T10:00:00-08:00
title: "Kubernetes 1.27: Introducing An API For Volume Group Snapshots"
date: 2023-05-08
slug: kubernetes-1-27-volume-group-snapshot-alpha
---

**Author:** Xing Yang (VMware)

Volume group snapshot is introduced as an Alpha feature in Kubernetes v1.27.
This feature introduces a Kubernetes API that allows users to take a crash consistent
snapshot for multiple volumes together. It uses a label selector to group multiple
PersistentVolumeClaims for snapshotting.
This new feature is only supported for CSI volume drivers.
This feature introduces a Kubernetes API that allows users to take crash consistent
snapshots for multiple volumes together. It uses a label selector to group multiple
`PersistentVolumeClaims` for snapshotting.
This new feature is only supported for [CSI](https://kubernetes-csi.github.io/docs/) volume drivers.

## What is Volume Group Snapshot
## An overview of volume group snapshots

Some storage systems provide the ability to create a crash consistent snapshot of
multiple volumes. A group snapshot represents “copies” from multiple volumes that
are taken at the same point-in-time. A group snapshot can be used either to rehydrate
new volumes (pre-populated with the snapshot data) or to restore existing volumes to
a previous state (represented by the snapshots).

## Why add Volume Group Snapshots to Kubernetes?
## Why add volume group snapshots to Kubernetes?

The Kubernetes volume plugin system already provides a powerful abstraction that
automates the provisioning, attaching, mounting, resizing, and snapshotting of block
Expand All @@ -30,9 +30,9 @@ and file storage.
Underpinning all these features is the Kubernetes goal of workload portability:
Kubernetes aims to create an abstraction layer between distributed applications and
underlying clusters so that applications can be agnostic to the specifics of the
cluster they run on and application deployment requires no “cluster specific” knowledge.
cluster they run on and application deployment requires no cluster specific knowledge.

There is already a [VolumeSnapshot API](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/177-volume-snapshot)
There is already a [VolumeSnapshot](/docs/concepts/storage/volume-snapshots/) API
that provides the ability to take a snapshot of a persistent volume to protect against
data loss or data corruption. However, there are other snapshotting functionalities
not covered by the VolumeSnapshot API.
Expand All @@ -45,25 +45,28 @@ If snapshots for the data volume and the logs volume are taken at different time
the application will not be consistent and will not function properly if it is restored
from those snapshots when a disaster strikes.

It is true that we can quiesce the application first, take an individual snapshot from
It is true that you can quiesce the application first, take an individual snapshot from
each volume that is part of the application one after the other, and then unquiesce the
application after all the individual snapshots are taken. This way we will get application
consistent snapshots.
However, application quiesce is time consuming. Sometimes it may not be possible to
quiesce an application. Taking individual snapshots one after another may also take
longer time compared to taking a consistent group snapshot. Some users may not want
to do application quiesce very frequently for these reasons. For example, a user may
want to run weekly backups with application quiesce and nightly backups without
application quiesce but with consistent group support which provides crash consistency
across all volumes in the group.
application after all the individual snapshots are taken. This way, you would get
application consistent snapshots.

However, sometimes it may not be possible to quiesce an application or the application
quiesce can be too expensive so you want to do it less frequently. Taking individual
snapshots one after another may also take longer time compared to taking a consistent
group snapshot. Some users may not want to do application quiesce very often for these
reasons. For example, a user may want to run weekly backups with application quiesce
and nightly backups without application quiesce but with consistent group support which
provides crash consistency across all volumes in the group.

## Kubernetes Volume Group Snapshots API

Kubernetes Volume Group Snapshots introduce [three new API objects](https://github.com/kubernetes-csi/external-snapshotter/blob/master/client/apis/volumegroupsnapshot/v1alpha1/types.go) for managing snapshots:
Kubernetes Volume Group Snapshots introduce [three new API
objects](https://github.com/kubernetes-csi/external-snapshotter/blob/master/client/apis/volumegroupsnapshot/v1alpha1/types.go)
for managing snapshots:

`VolumeGroupSnapshot`
: Created by a Kubernetes user (or perhaps by your own automation) to request
creation of a volume group snapshot for multiple volumes.
creation of a volume group snapshot for multiple persistent volume claims.
It contains information about the volume group snapshot operation such as the
timestamp when the volume group snapshot was taken and whether it is ready to use.
The creation and deletion of this object represents a desire to create or delete a
Expand All @@ -81,41 +84,50 @@ was created with a one-to-one mapping.
: Created by cluster administrators to describe how volume group snapshots should be
created. including the driver information, the deletion policy, etc.

The Volume Group Snapshot objects are defined as CustomResourceDefinitions (CRDs).
These three API kinds are defined as CustomResourceDefinitions (CRDs).
These CRDs must be installed in a Kubernetes cluster for a CSI Driver to support
volume group snapshots.

## How do I use Kubernetes Volume Group Snapshots

Volume Group Snapshot feature is implemented in the
Volume group snapshots are implemented in the
[external-snapshotter](https://github.com/kubernetes-csi/external-snapshotter) repository. Implementing volume
group snapshots meant adding or changing several components:

* Kubernetes Volume Group Snapshot CRDs
* Added new CustomResourceDefinitions for VolumeGroupSnapshot and two supporting APIs.
* Volume group snapshot controller logic is added to the common snapshot controller.
* Volume group snapshot validation webhook logic is added to the common snapshot validation webhook.
* Logic to make CSI calls is added to CSI Snapshotter sidecar controller.
* Adding logic to make CSI calls into the snapshotter sidecar controller.

The volume snapshot controller, CRDs, and validation webhook are deployed once per
cluster, while the sidecar is bundled with each CSI driver.

Therefore, it makes sense to deploy the volume snapshot controller, CRDs, and validation
webhook as a cluster addon. It is strongly recommended that Kubernetes distributors
webhook as a cluster addon. I strongly recommend that Kubernetes distributors
bundle and deploy the volume snapshot controller, CRDs, and validation webhook as part
of their Kubernetes cluster management process (independent of any CSI Driver).

### Creating a new group snapshot with Kubernetes

Once a VolumeGroupSnapshotClass object is defined and you have volumes you want to
snapshot together, you may create a new group snapshot by creating a VolumeGroupSnapshot
snapshot together, you may request a new group snapshot by creating a VolumeGroupSnapshot
object.

The source of the group snapshot specifies whether the underlying group snapshot
should be dynamically created or if a pre-existing VolumeGroupSnapshotContent
should be used. One of the following members in the source must be set.
should be used.

A pre-existing VolumeGroupSnapshotContent is created by a cluster administrator.
It contains the details of the real volume group snapshot on the storage system which
is available for use by cluster users.

One of the following members in the source of the group snapshot must be set.

* Selector - Selector is a label query over persistent volume claims that are to be grouped together for snapshotting. This labelSelector will be used to match the label added to a PVC.
* VolumeGroupSnapshotContentName - specifies the name of a pre-existing VolumeGroupSnapshotContent object representing an existing volume group snapshot.
* `selector` - a label query over PersistentVolumeClaims that are to be grouped
together for snapshotting. This labelSelector will be used to match the label
added to a PVC.
* `volumeGroupSnapshotContentName` - specifies the name of a pre-existing
VolumeGroupSnapshotContent object representing an existing volume group snapshot.

For dynamic provisioning, a selector must be set so that the snapshot controller can
find PVCs with the matching labels to be snapshotted together.
Expand All @@ -130,7 +142,8 @@ spec:
volumeGroupSnapshotClassName: csi-groupSnapclass
source:
selector:
group: myGroup
matchLabels:
group: myGroup
```
In the VolumeGroupSnapshot spec, a user can specify the VolumeGroupSnapshotClass which
Expand Down Expand Up @@ -187,7 +200,7 @@ snapshots that are part of a group snapshot.

## As a storage vendor, how do I add support for group snapshots to my CSI driver?

To implement the volume group snapshot feature, a CSI driver MUST:
To implement the volume group snapshot feature, a CSI driver **must**:

* Implement a new group controller service.
* Implement group controller RPCs: `CreateVolumeGroupSnapshot`, `DeleteVolumeGroupSnapshot`, and `GetVolumeGroupSnapshot`.
Expand All @@ -197,7 +210,6 @@ See the [CSI spec](https://github.com/container-storage-interface/spec/blob/mast
and the [Kubernetes-CSI Driver Developer Guide](https://kubernetes-csi.github.io/docs/)
for more details.

Although Kubernetes poses as little prescriptive on the packaging and deployment of
a CSI Volume Driver as possible, it provides a suggested mechanism to deploy a
containerized CSI driver to simplify the process.

Expand All @@ -215,8 +227,11 @@ The external-snapshotter watches the Kubernetes API server for the
The alpha implementation of volume group snapshots for Kubernetes has the following
limitations:

* Does not support reverting an existing PVC to an earlier state represented by a snapshot that is part of a group snapshot (only supports provisioning a new volume from a snapshot).
* No application consistency guarantees beyond any guarantees provided by the storage system (e.g. crash consistency).
* Does not support reverting an existing PVC to an earlier state represented by
a snapshot (only supports provisioning a new volume from a snapshot).
* No application consistency guarantees beyond any guarantees provided by the storage system
(e.g. crash consistency). See this [doc](https://github.com/kubernetes/community/blob/master/wg-data-protection/data-protection-workflows-white-paper.md#quiesce-and-unquiesce-hooks)
for more discussions on application consistency.

## What’s next?

Expand All @@ -227,11 +242,11 @@ replication group, volume placement, application quiescing, changed block tracki

## How can I learn more?

The design spec for the volume group snapshot feature is [here](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/3476-volume-group-snapshot).

The code repository for volume group snapshot APIs and controller is [here](https://github.com/kubernetes-csi/external-snapshotter).

Check out additional documentation on the group snapshot feature [here](https://kubernetes-csi.github.io/docs/).
- The [design spec](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/3476-volume-group-snapshot)
for the volume group snapshot feature.
- The [code repository](https://github.com/kubernetes-csi/external-snapshotter) for volume group
snapshot APIs and controller.
- CSI [documentation](https://kubernetes-csi.github.io/docs/) on the group snapshot feature.

## How do I get involved?

Expand Down

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